jwmw
Primary domain and DOMAIN are different things.
The answer has been provided, use the workgroup name from server manager, as that is your DOMAIN name for joining a trusted network otherwise called a DOMAIN.
Re the primary or main domain of gotrain.local, you should not use this type of Windows style domain (web URL) naming parlance on SME server, as it will cause you problems in the future, especially with email & reverse lookups etc. Other servers will not trust your server as the domain name is not a resolvable URL. Use gotrain.com or whatever your registered main web domain name is.
This is configurable in the admin console ie login as admin at the command prompt which then displays the admin console menu, then select Configure this server, step through the options changing the primary or main domain name, then save & reboot.
For DOMAIN name (referring to a trusted DOMAIN) & same as workgroup name in SME server, only use a single word without punctuation marks etc eg gotrain or gserver (not gotrain.local or gserver.local).
This is configurable in server manager.